1. Field of the Invention
This invention relates to a process for bleaching mechanical wood pulp with sodium hydrosulfite as part of a refining process.
2. Description of the Prior Art
In a typical conventional pulp refining process, wood chips or the like are subjected to two or more refining stages, in which they are ground mechanically by rotating grinding wheels or discs and then to a bleaching stage to remove chromophores and increase the brightness of the pulp.
The first refining stage is generally carried out using steam of an elevated pressure, suitably 100-200 KPa. The subsequent refining stages can be carried out at atmospheric pressure. The resulting pulp is then subjected to post-bleaching in a tower or chest, at low to medium consistency.
The most commonly used pulp bleaching agents are hydrogen peroxide, H.sub.2 O.sub.2, and sodium hydrosulfite, Na.sub.2 S.sub.2 O.sub.4, also known as sodium dithionite. Whilst the peroxide generally provides greater brightness gains, it is relatively expensive and the hydrosulfite is therefore more commonly utilized. This compound cannot however be used at high concentration since its decomposition products tend to act as catalysts, promoting the decomposition of the hydrosulfite and inhibiting its bleaching activity.
Barton and Treadway, in Pulp Paper 53, No. 6. pp. 180-181 propose feeding a part of the hydrosulfite to a refining stage before the pulp reaches the bleaching tower. The elevated temperature (typically 145.degree. F., 65.5.degree. C.) and high pulp consistency were found to offer considerable advantages, as was the absence of air in a pressurised refiner. Rather than increase the total amount of hydrosulfite used, Barton and Treadway reduced the hydrosulfite concentration in the bleaching tower, splitting the total between the refiner and the tower.
Melzer and Auhorn, in a paper given to the Wood Pulp Symposium in Munich in 1985, showed how the total hydrosulfite input could be reduced by feeding the greater part of the hydrosulfite used to the first stage of a two-stage refining process at pH 6, and adding the rest to the refined pulp before it entered a bleaching tower. This also gave a marked saving in energy consumption to produce the same mechanical pulp properties, or improved strength characteristics for the same energy input. No improvement in brightness was noted, however.